Vincent O. Boer

31P MRSI and 1H MRS at 7 T: initial results in human breast cancer

This study demonstrates the feasibility of the noninvasive determination of important biomarkers of human (breast) tumor metabolism using high‐field (7‐T) MRI and MRS. 31P MRSI at this field strength was used to provide a direct method for the in vivo detection and quantification of endogenous biomarkers. These encompass phospholipid metabolism, phosphate energy metabolism and intracellular pH. A double‐tuned, dual‐element transceiver was designed with focused radiofrequency fields for unilateral breast imaging and spectroscopy tuned for optimized sensitivity at 7 T. T1‐weighted three‐dimensional MRI and 1H MRS were applied for the localization and quantification of total choline compounds. 31P MRSI was obtained within 20 min per subject and mapped in three dimensions over the breast with pixel volumes of 10 mL. The feasibility of monitoring in vivo metabolism was demonstrated in two patients with breast cancer during neoadjuvant chemotherapy, validated by ex vivo high‐resolution magic angle spinning NMR and compared with data from an age‐matched healthy volunteer. Concentrations of total choline down to 0.4 mM could be detected in the human breast in vivo. Levels of adenosine and other nucleoside triphosphates, inorganic phosphate, phosphocholine, phosphoethanolamine and their glycerol diesters detected in glandular tissue, as well as in tumor, were mapped over the entire breast. Altered levels of these compounds were observed in patients compared with an age‐matched healthy volunteer; modulation of these levels occurred in breast tumors during neoadjuvant chemotherapy. To our knowledge, this is the first comprehensive MRI and MRS study in patients with breast cancer, which reveals detailed information on the morphology and phospholipid metabolism from volumes as small as 10 mL. This endogenous metabolic information may provide a new method for the noninvasive assessment of prognostic and predictive biomarkers in breast cancer treatment. Copyright

Dynamic multi-coil shimming of the human brain at 7 T.

High quality magnetic field homogenization of the human brain (i.e. shimming) for MR imaging and spectroscopy is a demanding task. The susceptibility differences between air and tissue are a longstanding problem as they induce complex field distortions in the prefrontal cortex and the temporal lobes. To date, the theoretical gains of high field MR have only been realized partially in the human brain due to limited magnetic field homogeneity. A novel shimming technique for the human brain is presented that is based on the combination of non-orthogonal basis fields from 48 individual, circular coils. Custom-built amplifier electronics enabled the dynamic application of the multi-coil shim fields in a slice-specific fashion. Dynamic multi-coil (DMC) shimming is shown to eliminate most of the magnetic field inhomogeneity apparent in the human brain at 7 T and provided improved performance compared to state-of-the-art dynamic shim updating with zero through third order spherical harmonic functions. The novel technique paves the way for high field MR applications of the human brain for which excellent magnetic field homogeneity is a prerequisite.

GABA and glutamate in schizophrenia: A 7 T 1H-MRS study

Schizophrenia is characterized by loss of brain volume, which may represent an ongoing pathophysiological process. This loss of brain volume may be explained by reduced neuropil rather than neuronal loss, suggesting abnormal synaptic plasticity and cortical microcircuitry. A possible mechanism is hypofunction of the NMDA-type of glutamate receptor, which reduces the excitation of inhibitory GABAergic interneurons, resulting in a disinhibition of glutamatergic pyramidal neurons. Disinhibition of pyramidal cells may result in excessive stimulation by glutamate, which in turn could cause neuronal damage or death through excitotoxicity. In this study, GABA/creatine ratios, and glutamate, NAA, creatine and choline concentrations in the prefrontal and parieto-occipital cortices were measured in 17 patients with schizophrenia and 23 healthy controls using proton magnetic resonance spectroscopy at an ultra-high magnetic field strength of 7 T. Significantly lower GABA/Cr ratios were found in patients with schizophrenia in the prefrontal cortex as compared to healthy controls, with GABA/Cr ratios inversely correlated with cognitive functioning in the patients. No significant change in the GABA/Cr ratio was found between patients and controls in the parieto-occipital cortex, nor were levels of glutamate, NAA, creatine, and choline differed in patients and controls in the prefrontal and parieto-occipital cortices. Our findings support a mechanism involving altered GABA levels distinguished from glutamate levels in the medial prefrontal cortex in schizophrenia, particularly in high functioning patients. A (compensatory) role for GABA through altered inhibitory neurotransmission in the prefrontal cortex may be ongoing in (higher functioning) patients with schizophrenia.

Direct B0 field monitoring and real-time B0 field updating in the human breast at 7 Tesla

Large dynamic fluctuations of the static magnetic field (B0) are observed in the human body during MR scanning, compromising image quality and detection sensitivity in several MR imaging and spectroscopy sequences. Partially, these dynamic B0 fluctuations are due to physiological motion such as breathing, but other sources of temporal B0 field fluctuations are also present in the MR system (e.g., eddy currents). Especially at ultrahigh field (≥7 T), the increased susceptibility effects lead to large B0 field variations over time. Direct measurement and correction of these temporal field variations of up to 70 Hz will lead to a significant reduction of artifacts and improved measurement stability/reproducibility. For direct measurement of the temporally changing B0 field, a simple field probe was developed, that was placed in proximity to the tissue of interest. In this work, it is shown how such a field probe system can be used to monitor temporal B0 field variations in the human body during MRI and magnetic resonance spectroscopy. Furthermore, it is shown how the acquired temporal B0 field information can drive a dynamic shim module to directly correct the B0 magnetic field in real time. Magn Reson Med, 2012.

7-T 1H MRS with adiabatic refocusing at short TE using radiofrequency focusing with a dual-channel volume transmit coil

In vivo MRS of the human brain at ultrahigh field allows for the identification of a large number of metabolites at higher spatial resolutions than currently possible in clinical practice. However, the in vivo localization of single‐voxel spectroscopy has been shown to be challenging at ultrahigh field because of the low bandwidth of refocusing radiofrequency (RF) pulses. Thus far, the proposed methods for localized MRS at 7 T suffer from long TE, inherent signal loss and/or a large chemical shift displacement artifact that causes a spatial displacement between resonances, and results in a decreased efficiency in editing sequences. In this work, we show that, by driving a standard volume coil with two RF amplifiers, focusing the B  1+ field in a certain location and using high‐bandwidth adiabatic refocusing pulses, a semi‐LASER (semi‐localized by adiabatic selective refocusing) localization is feasible at short TE in the human brain with full signal acquisition and a low chemical shift displacement artifact at 7 T. Copyright

Efficient spectral editing at 7 T: GABA detection with MEGA-sLASER.

At high field (7 T) spectral editing of γ‐aminobutyric acid with MEGA‐point‐resolved spectroscopy is inefficient due to the large chemical shift displacement error. In this article, a new pulse sequence is designed which has minimal chemical shift displacement error to perform an efficient spectral editing of the γ‐aminobutyric acid 3.0 ppm resonance at 7 T. The sequence consists of the conventional MEGA editing pulses and a semi‐localized by adiabatic selective refocusing sequence. Phantom and in vivo measurements demonstrated an efficient detection of γ‐aminobutyric acid. Using ECG triggering, excellent in vivo performance of the MEGA‐semi‐localized by adiabatic selective refocusing (MEGA‐sLASER) provided well‐resolved γ‐aminobutyric acid signals in 27 mL volumes in the human brain at an echo time of 74 ms within a relatively short acquisition time (5 min). Furthermore, the high efficiency of the MEGA‐sLASER was demonstrated by acquiring small volumes (8 mL) at an echo time of 74 ms, as well as long echo time measurements (222 ms in 27 mL volume). Magn Reson Med, 2012.

Multislice 1H MRSI of the human brain at 7 T using dynamic B0 and B1 shimming

Proton MR spectroscopic imaging of the human brain at ultra‐high field (≥7 T) is challenging due to increased radio frequency power deposition, increased magnetic field B0 inhomogeneity, and increased radio frequency magnetic field inhomogeneity. In addition, especially for multislice sequences, these effects directly inhibit the potential gains of higher magnetic field and can even cause a reduction in data quality. However, recent developments in dynamic B0 magnetic field shimming and dynamic multitransmit radio frequency control allow for new acquisition strategies. Therefore, in this work, slice‐by‐slice B0 and B1 shimming was developed to optimize both B0 magnetic field homogeneity and nutation angle over a large portion of the brain. Together with a low‐power water and lipid suppression sequence and pulse‐acquire spectroscopic imaging, a multislice MR spectroscopic imaging sequence is shown to be feasible at 7 T. This now allows for multislice metabolic imaging of the human brain with high sensitivity and high chemical shift resolution at ultra‐high field. Magn Reson Med, 2012.

Diffusion-weighted MR neurography of the sacral plexus with unidirectional motion probing gradients

BackgroundThis technical note introduces diffusion-weighted (DW) MR neurography (MRN) of the sacral plexus with unidirectional motion probing gradients (MPGs).MethodsThis is compared with DW MRN with three-directional and six-directional MPGs.Results and conclusionThis paper indicates that DW MRN of the sacral plexus should be performed with unidirectional MPGs.

High‐field MRS of the human brain at short TE and TR

In vivo MRS of the human brain at 7 tesla allows identification of a large number of metabolites at higher spatial resolutions than currently possible at lower field strengths. However, several challenges complicate in vivo localization and artifact suppression in MRS at high spatial resolution within a clinically feasible scan time at 7 tesla. Published MRS sequences at 7 tesla suffer from long echo times, inherent signal‐to‐noise ratio (SNR) loss, large chemical shift displacement artifacts or long repetition times because of excessive radiofrequency (RF) power deposition. In the present study a pulse‐acquire sequence was used that does not suffer from these high field drawbacks. A slice selective excitation combined with high resolution chemical shift imaging for in‐plane localization was used to limit chemical shift displacement artifacts. The pulse‐acquire approach resulted in a very short echo time of 1.4 ms. A cost function guided shimming algorithm was developed to constrain frequency offsets in the excited slice, therefore adiabatic frequency selective suppression could be employed to minimize artifacts from high intensity lipids and water signals in the excited slice. The high sensitivity at a TR of 1 s was demonstrated both on a supraventricular slice as well as in an area very close to the skull in the frontal cortex at a nominal spatial resolution of 0.25 cc within a feasible scan time. Copyright

Ultra high spatial and temporal resolution breast imaging at 7T

There is a need to obtain higher specificity in the detection of breast lesions using MRI. To address this need, Dynamic Contrast‐Enhanced (DCE) MRI has been combined with other structural and functional MRI techniques. Unfortunately, owing to time constraints structural images at ultra‐high spatial resolution can generally not be obtained during contrast uptake, whereas the relatively low spatial resolution of functional imaging (e.g. diffusion and perfusion) limits the detection of small lesions. To be able to increase spatial as well as temporal resolution simultaneously, the sensitivity of MR detection needs to increase as well as the ability to effectively accelerate the acquisition. The required gain in signal‐to‐noise ratio (SNR) can be obtained at 7T, whereas acceleration can be obtained with high‐density receiver coil arrays. In this case, morphological imaging can be merged with DCE‐MRI, and other functional techniques can be obtained at higher spatial resolution, and with less distortion [e.g. Diffusion Weighted Imaging (DWI)]. To test the feasibility of this concept, we developed a unilateral breast coil for 7T. It comprises a volume optimized dual‐channel transmit coil combined with a 30‐channel receive array coil. The high density of small coil elements enabled efficient acceleration in any direction to acquire ultra high spatial resolution MRI of close to 0.6 mm isotropic detail within a temporal resolution of 69 s, high spatial resolution MRI of 1.5 mm isotropic within an ultra high temporal resolution of 6.7 s and low distortion DWI at 7T, all validated in phantoms, healthy volunteers and a patient with a lesion in the right breast classified as Breast Imaging Reporting and Data System (BI‐RADS) IV. Copyright